Network Working Group
Internet-Draft TCI
Intended status: Experimental March 6, 2019
Expires: September 7, 2019
Fake Server Name Indication
draft-belyavskiy-fakesni-02
Abstract
The document provides a specification of the Fake Server Name
Indication. Being implemented, the Fake SNI specification provides a
way to cheat the monitoring solutions without providing any
additional information to external observers.
Status of This Memo
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This Internet-Draft will expire on September 7, 2019.
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1. Introduction
Many DPI solutions use SNI information as a criterion to filter
connection to various sites. Though Encrypted SNI makes impossible
to read the SNI value, there is information [1] that absence of SNI
looks suspicious itself and all communications are blocked.
This specification introduces a way to provide a value of SNI treated
by TLS server as an alias to one of the names known by server but not
matching the possibly suspicious hostname.
This specification does not save from DPI solutions but it provides
one more loophole to cheat them.
2. Fake SNI design goals
The solution specified in this document is inspired by the design of
Encrypted SNI. It is fully-compatible with current TLS
specifications. As it does not make much sense to use it with TLS
1.0-1.2, where the original host name will be provided unencrypted in
the certificate, in case of TLS 1.3 the certificate is delivered
encrypted.
The provider publishes a name matching the target name to be provided
in the clear text. This document defines a publication mechanism
using DNS, but other mechanisms are also possible.
When a client wants to establish a TLS connection to a domain served
by a Fake SNI-supporting provider, it replaces the value in
"server_name" extension in the ClientHello with the value obtained by
transport. The provider can then find out the desired name from its
configuration and either establish the connection with the desired
host or reject it.
3. Definitions
Original name - the hostname of service that is subject to hide.
Fake name - the hostname specified by server and sent by client to
indicate intention to connect to host with original name.
4. Fake SNI indication
Fake SNI information is published in DNS via TXT RR. For example,
the Fake SNI record for domain example.com may look like
_fakesni.example.com. 60S IN TXT "myfakerecord.com IP"
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where IP address may be omitted. If present, it MUST match an IP
address specified in A/AAAA record for the domain. Specifying IP
address for a specific fake name may help in case when a service is
hosted using more than one CDN.
The fake name specified in the Fake SNI RR MUST identify the original
hostname it is valid for. Fake names for different hosts on the same
IP address MUST be different to distinguish the original names.
5. Server behaviour
On receiveing the value of known Fake SNI in the TLS ClientHello
server MUST return the certificate matching the original hostname.
Otherwise server SHOULD abort the connection.
6. Client behaviour
Client MAY use the Fake SNI record as fallback if connecting using
ESNI is blocked. In this case client initiates normal TLS connection
specifying the value from Fake SNI record in the server_name
extension. If the certificate received from server does not match
the original hostname, the client MUST abort the connection.
Otherwise the client MUST follow the normal process of TLS handshake.
7. Operational considerations
Depending on the DPI modus operandi it may make sense to provide a
valid fake name (e.g. from deep-level subdomain) resolving to the
same IPs as original hostname does. If DPI tries to resolve the fake
name, such behaviour will make distinguishing between real and fake
names difficult.
8. Security considerations
As Fake SNI can be used in TLS 1.2, it does not provide any problems
to DPI because in this case the original hostname is available in
clear text in server certificate. TLS 1.3 encrypts the Certificate
message, so it is RECOMMENDED to use Fake SNI together with TLS 1.3.
To strengthen the protection, it's recommended to obtain _fakesni RR
via DoT or DoH.
As DPI solutions are able to obtain the DNS _fakesni records as
legitimate clients do, it is RECOMMENDED to set reasonable TTL values
for the _fakesni records. Also it is RECOMMENDED to use such values
of fake names that are syntactically correct domain names. Otherwise
DPI can recognise the fake names as fake ones.
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9. Current version of the specification
The current version of the specification is available at GitHub
repository [2].
10. References
10.1. URIs
[1] https://mailarchive.ietf.org/arch/msg/tls/
WiT3oEh6PO96mm0z28BNMp0YgGs
[2] https://github.com/beldmit/fakesni/
Author's Address
Dmitry Belyavskiy
Cryptocom LTD
Kedrova st, 14/2
Moscow 117218
RU
Email: beldmit@gmail.com
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